Hemicelluloses delignification

During heating the pH dropped below 4 due to the formation of acetic acid. The acid caused hydrolysis of hemicelluloses and formation of furfural. It is thus possible that the delignification was caused by hydrolytic cleavage of interunit linkages. Figure 3 shows that the dissolved lignins... 

The sulfite process for delignification of wood in the paper industry involves heating with acidic sulfite solutions at elevated temperatures under pressure. Under such conditions, oxidation occurs to a certain extent, and presumably, the sulfite is partly reduced to sulfide. The composition of the liquors resulting from the treatment of cellulose and hemicelluloses, and alditols with sulfite in acid medium has been described (Ref. 1, p. 1150). 

Understanding the MnP and LiP mechanism is important not only from a fundamental point of view but also because delignification of wood chips leaves behind pulp, the fibrous polysaccharides (hemicellulose and cellulose) that... 

A major problem in the commercialization of this potential is the inherent resistance of lignocellulosic materials toward conversion to fermentable sugars (4). To improve the efficiency of enzymatic hydrolysis, a pretreatment step is necessary to make the cellulose fraction accessible to cellulase enzymes. Delignification, removal of hemicellulose, and decreasing the crystallinity of cellulose produce more accessible surface area for cellulase enzymes to react with cellulose (5). 

Ideally, the purpose of delignification is to remove the lignin at least from the middle lamella as selectively as possible. In reality, however, attack on the polysaccharides cannot be avoided and this results in depolymerization of polysaccharides and losses of hemicellulose. 

Non-endospermic tissues of grasses are often subjected not only to delignification but to other treatments before the intentional extraction of hemicellulosic materials. It is not always clear that a useful purpose is served by some of these treatments, or that they do, or only do, what is wanted. The fact that a procedure has often been used does not alone justify its routine adoption or retention. As mentioned, enzymes may be inactivated by hot, aqueous alcohol78,79 in which oligo-... 

Hemicelluloses have traditionally been defined by extraction procedures, i. e., hemicelluloses are those polysaccharides extracted by alkaline solutions from plant tissues after removal of low-molecular-weight substances with hot aqueous alcohol, removal of waxes and other lipid-soluble substances, delignification, and removal of pectin with an aqueous solution of a calcium ion chelator. The most abundant hemicelluloses in the primary cell walls of dicotyledons are xyloglucans. Xyloglucans have a backbone chain whose chemical stmcture is identical to that of cellulose. About 75% of the 8-D-glucopyranosyl units in that chain are substituted... 

In an attempt to improve the yield of hemicelluloses. Sands and Nutter subjected mesquite (Prosopis julifiora) wood to alternate extraction with alkali and delignification with chlorine and aqueous ammonia. The resulting fractions all consisted of a xylan whose average composition corresponded to one methoxyl group, one uronic acid residue, and ten xylose residues, or exactly the same composition as had been noted by O Dwyer for her oak xylan." ... 

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